The present invention relates to fiber-optic communications, and more particularly, to optical amplifiers and methods for manufacturing optical amplifiers for use in optical communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometer fiber-optic links. A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. The erbium-doped fiber amplifier stages increase the strength of the optical signals being transmitted over the fiber-optic links.
There is typically a lot-to-lot variation in the gain characteristics of doped erbium fiber. There are also typically variations in the losses associated with passive optical amplifier components. These variations and variations that arise in the losses associated with assembling erbium fiber coils and other components to form a completed amplifier adversely affect the ability to manufacture amplifiers with operating characteristics that precisely match design expectations.
It is an object of the present invention to provide methods for assembling optical amplifier systems with operating characteristics that match design expectations.
It is also an object of the present invention to provide optical amplifier arrangements that may be manufactured to match design expectations.
These and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers arrangements and methods for manufacturing optical amplifiers that allow amplifiers to be fabricated to match design expectations.
With one approach, optical amplifiers may be manufactured by assembling the passive optical components in the amplifier, before assembling the active optical components such as the doped fiber coils. Passive losses may then be characterized and used to calculate the lengths of the fibers that should be used in the amplifier gain stages.
Following corrections to the nominal doped-fiber lengths based on the measured passive losses, the passive and active components of the amplifier may be assembled and characterized. An optical measurement system may be used to measure the performance of the amplifier assembly at this stage of the manufacturing process. The measurement system may use two or more optical wavelengths, so that any tilt in the gain spectrum of the amplifier assembly may be characterized.
Final corrections may be made to the amplifier assembly based on the results of the gain tilt measurements. For example, the lengths of one or more of the doped fibers may be adjusted by adding or removing segments or doped fiber. If desired, such length adjustments may be made to fiber coils in the mid-stage portion of the amplifier, so that the impact on the operating characteristics of the amplifier are minimized. If the fiber coil length is to be shortened, an appropriate segment of fiber may be removed from a mid-stage coil. If the fiber coil length is to be lengthened, an additional segment of doped fiber may be spliced to the mid-stage coil.
If desired, the process of adjusting the length of the fiber coils may be anticipated when constructing the initial amplifier assembly. This may allow, for example, the final fiber length correction to always be a fiber-length reduction or to always be a fiber-length addition. The amplifier may also be configured so that some fiber-length changes will involve length reductions and some fiber-length changes will involve length additions.
The temperature of the fiber coils used in the amplifier may be regulated using a temperature-controlled fiber coil housing. If desired, segments of fiber that are added to the amplifier after the coils have been mounted in the housing may be spliced into the main fiber path of the amplifier at a location outside of the housing.
For some applications it may be desirable to assemble all of the passive and active optical components of the amplifier at once based on a nominal design. Length changes may then be made to the fiber coils to correct for variations from the design expectation of the amplifier.
The amplifier may use a variable optical attenuator. The amount of attenuation contributed by the variable optical attenuator can be used to adjust the tilt of the amplifier gain spectrum.
The amplifier may use a nominal design in which the variable optical attenuator is set to produce a loss of 0 dB. The performance of the amplifier may then be optimized during the manufacturing process by adjusting the length of one or more of the active fiber coils or by adjusting the loss setting of the variable optical attenuator.
Another approach involves using a nominal design in which the variable optical attenuator is set to produce a small loss (e.g., 1-2 dB). After the performance of the amplifier assembly is characterized, the setting of the variable optical attenuator may be adjusted to produce more or less attenuation and thereby change the gain tilt to match the design expectation for the amplifier. With this approach, it may be possible to manufacture the amplifier within design tolerances without changing the lengths of the fiber coils or without changing the lengths of the fiber coils as much as would otherwise be required.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.